[Exim] restricting a router to specific sender_host_address

I am surprised I have not been able to work out how to do what seems a quite simple
test in a router. I want to restrict a router to SMTP connections from hosts
external to my network.

I already have:

hostlist local_ip = 144.173.0.0/16 : 172.16.0.0/16 : 127.0.0.1

and use that in ACLs with

hosts = +local_ip

However, I just can't see how I user +local_ip to restrict a router to be invoked
olny if "hosts = ! +local_ip". "hosts" is not a router option and I can't work out
how to user $sender_host_address and +local_ip in a condition.

Sorry if this is really simple, which I suspect, but I just can't find it in the
documentation.

Phil.
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Phil Chambers (postmaster@???)
University of Exeter